How much water should be added to 1 L of 2 M solution to make the final molarity 0.8 M?
Answer and explanation
Correct answer: 1.5 L
Dilution preserves the moles of solute, so use M1V1 = M2V2. Initially, n = 2 mol L−1 × 1 L = 2 mol. To obtain a final concentration of 0.8 M, the required final volume is V2 = 2 mol ÷ 0.8 mol L−1 = 2.5 L. The original volume was 1.0 L, so water added = 2.5 − 1.0 = 1.5 L. Hence option B is correct; 2.5 L is the final volume, not the added amount.
Frequently asked questions
What is the correct answer to this question?
1.5 L
Why is this the correct answer?
Dilution preserves the moles of solute, so use M1V1 = M2V2. Initially, n = 2 mol L−1 × 1 L = 2 mol. To obtain a final concentration of 0.8 M, the required final volume is V2 = 2 mol ÷ 0.8 mol L−1 = 2.5 L. The original volume was 1.0 L, so water added = 2.5 − 1.0 = 1.5 L. Hence option B is correct; 2.5 L is the final volume, not the added amount.
Which subject and chapter does this question cover?
This is a Class 11 Chemistry question. Chapter: Some Basic Concepts of Chemistry. Topic: Mole concept and molar mass.
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